Brought to you in part by:

.


Tools and Supplies at Eastwood

  Mustangsandmore Forums
  '64 1/2 to '73 -- The Classic Mustang
  Carb selection

Post New Topic  Post A Reply
profile | register | preferences | faq | search

UBBFriend: Email This Page to Someone! next newest topic | next oldest topic
Author Topic:   Carb selection
BAD67FUN
Journeyman

Posts: 51
From:
Registered: Feb 2002

posted 03-06-2002 04:41 PM     Click Here to See the Profile for BAD67FUN     Edit/Delete Message   Reply w/Quote
I'm planning on adding the Edelbrock Air Gap with the Comp 270H cam. Car has headers, C4, and will sport 3.80 gears. Everything else is stock for now. I'm guessing 600 cfm will be okay. But why not get some opinions that include part numbers? Car does not need to pass emissions. I'm sitting here with a Summit and Jegs catalog in front of me. So please help me pick. One more ingredient.... it's a street car. Thanks

IP: Logged

Hemikiller
Gearhead

Posts: 256
From: Clinton, CT
Registered: Feb 2002

posted 03-06-2002 05:44 PM     Click Here to See the Profile for Hemikiller   Click Here to Email Hemikiller     Edit/Delete Message   Reply w/Quote
I'm assuming that it's a 289/302? I'd go with at least a Holley 650 Mech secondary carb. Don't buy the crapper 1850 600 vac secondary carb, they will cost you as much to hop up as it would to be to buy a 650 DP. If you plan to hop it up later with more motor, etc, get a 700. Remember, you can always let OFF the gas a little (or use a throttle stop) but you cant add cfm to a carb. My 351C's all had 800 and 850 double pumpers on them. You have plenty of gear, and a 67 is a reasonably light car.

IP: Logged

Calcat
Journeyman

Posts: 75
From: Los Angeles, CA
Registered: Feb 2002

posted 03-07-2002 02:54 AM     Click Here to See the Profile for Calcat   Click Here to Email Calcat     Edit/Delete Message   Reply w/Quote
Street car - get the smallest carb you can that will adequately feed the engine. crisper throttle response. I would look at the 625 Demon. Personally, I see no advantage to a dbl pumper on the street if driveability is of any concern.

IP: Logged

70SportsRoof
Gearhead

Posts: 471
From: Mesa, AZ...for now....
Registered: Dec 2001

posted 03-07-2002 04:20 AM     Click Here to See the Profile for 70SportsRoof   Click Here to Email 70SportsRoof     Edit/Delete Message   Reply w/Quote
Too big of a carb will give you sh**ty performance. A 650-700 would be about right for your application. In the 2002 Edelbrock catalog they recommend between 600-800 CFM for the Air Gap 302 manifold, and they say 800 for the Air Gap 351W. So, I would say for a 302 a 650-700 would be about right for your car. Try looking through Edelbrock's website for some helpful info. They have section called tech info that has some help for selecting a proper CFM carb for your app.
www.edelbrock.com

------------------
1970 Deluxe SportsRoof (63B),'66 fastback 289-4 spd(going to a new home),(2)46 Ford trucks, 47 Ford truck, 57 Ford Fairlane 500 4dr, '72 Olds Vista Cruiser(up for sale soon), '88 Bronco II(for sale)...running out of room, selling some of my projects

IP: Logged

Daniel Jones
Gearhead

Posts: 420
From: St. Louis, MO
Registered: Aug 99

posted 03-07-2002 11:13 AM     Click Here to See the Profile for Daniel Jones   Click Here to Email Daniel Jones     Edit/Delete Message   Reply w/Quote
Please ignore the following. It's just a test. If it works, I'll post some carb sizing info.

Dan Jones

................DISP RPM
........CFM = ----- * ---- * VE
.................2 1728

IP: Logged

Daniel Jones
Gearhead

Posts: 420
From: St. Louis, MO
Registered: Aug 99

posted 03-07-2002 11:15 AM     Click Here to See the Profile for Daniel Jones   Click Here to Email Daniel Jones     Edit/Delete Message   Reply w/Quote
Hmmm, I thought the reply window didn't recognize blank spaces and you needed the
.... for formatting. Let's see:


DISP RPM
CFM = ----- * ---- * VE
2 1728

IP: Logged

Daniel Jones
Gearhead

Posts: 420
From: St. Louis, MO
Registered: Aug 99

posted 03-07-2002 11:17 AM     Click Here to See the Profile for Daniel Jones   Click Here to Email Daniel Jones     Edit/Delete Message   Reply w/Quote
Yup, blanks screw up the formatting. Is there someway to preserve formatting? The .... sort of work but are visible.
Thanks,

Dan Jones

IP: Logged

SteveLaRiviere
Administrator

Posts: 30097
From: Saco, Maine
Registered: May 99

posted 03-07-2002 01:05 PM     Click Here to See the Profile for SteveLaRiviere   Click Here to Email SteveLaRiviere     Edit/Delete Message   Reply w/Quote
Yes, use the code function. [*code] before and [/code] after. {remove the * of course.}

------------------

'70 Mustang Mach 1 M code 351C 4V/FMX/3.25 open
'72 Mustang Sprint Coupe 351C 4V/FMX/4.30 Trac Lok
'94 F-150 XL 5.8L/E4OD/3.55 Limited Slip

MCA Member # 47773

An optimist thinks that this is the best possible world. A pessimist fears that this is true.

IP: Logged

Daniel Jones
Gearhead

Posts: 420
From: St. Louis, MO
Registered: Aug 99

posted 03-07-2002 01:55 PM     Click Here to See the Profile for Daniel Jones   Click Here to Email Daniel Jones     Edit/Delete Message   Reply w/Quote
code:
                DISP * RPM * VE
CFM = ---------------
3456

IP: Logged

Daniel Jones
Gearhead

Posts: 420
From: St. Louis, MO
Registered: Aug 99

posted 03-07-2002 01:59 PM     Click Here to See the Profile for Daniel Jones   Click Here to Email Daniel Jones     Edit/Delete Message   Reply w/Quote
Okay, let's give this a try. Here's some carb sizing info I put together a while back:
code:

Selecting the Proper Size Carb by Dan Jones

There are a number of sizing formulas that can be used to estimate the best
flow rating for a given application. The standard equation relating engine
size and required carb flow is:


where:

DISP = engine displacement in cubic inches
CFM = required carb flow in cubic feet per minute
RPM = maximum engine speed in revolutions per minute
VE = volumetric efficiency (dimensionless, 1.0 = 100%)
1728 = conversion factor between cubic inches and cubic feet
= 12*12*12
2 = conversion factor for four stroke engine

This equation can be simplified to:

DISP * RPM * VE
CFM = ---------------
3456

Note this sizing formula is simply a relationship between cylinder volume and
the flow required to fill that volume at a given engine speed. Also note,
for a four stroke engine, displacement is divided by two because an intake
stroke occurs every other revolution. Another implicit assumption is that
the carb is mounted on a plenum style intake. Independent runner intakes will
require much different sizing. While it's easy to determine displacement and
maximum rpm, you'll probably have to guess at the third variable, volumetric
efficiency (VE), unless you have access to a dyno. Volumetric efficiency is

a simply a measure of how efficiently an engine fills its cylinders. A stock,
low performance, street engine may have a VE between 0.7 and 0.8. High
performance street engines may fall between 0.8 and 1.0, while highly tuned
race engines can have VE's exceeding 1.0, perhaps as high as 1.25.

One other thing to understand when using the formula above is that a carb will
only flow in the presence of a pressure differential. On one side of the carb
there is atmospheric pressure and on the other side is manifold pressure
(usually referred to as manifold vacuum since it is typically lower than
atmospheric pressure). Since engines vary in their manifold vacuum
characteristics, a standardized pressure differential was established to
provide a meaningful comparison for different carbs. Before this standard,
venturi size was used for comparison. The standard for four barrel carbs is
a pressure differential equal to 1.5 inches of mercury (Hg). What this means
is a 4 barrel carb rated at 500 CFM will flow 500 CFM of air, at wide open
throttle, when a pressure differential of 1.5 In Hg is applied across it.
When installed on an engine, this same carb may flow more or less. Two barrel
carbs are usually rated at a different pressure differential (3.0 In Hg). The
reason for this is primarily historical. When 4 barrel carbs first came into
popular use, the vacuum pumps used to rate 2 barrel carbs were unable to pull
the same pressure differential across a 4 barrel carb, so 4 barrels were rated
at a lower pressure drop.

Flow ratings from one standard can be related to flow ratings from another
standard. For 2 and 4 barrel carbs:

Flow @ 1.5 In Hg = CFM Rating @ 3.0 In Hg
----------------------
SQRT(3.0/1.5)

Which is approximately:

Flow @ 1.5 In Hg = CFM Rating @ 3.0 In Hg
----------------------
1.414

This relationship is derived from the fact that, for incompressible flow, the
volumetric flow rate through a venturi is proportional to the square root of
the pressure differential:

Q = K1*A2*SQRT(2*Gc/Rho)*SQRT(P1-P2)

or more simply:

Q = K2*SQRT(P1-P2)

where:

Q = volumetric flow rate
K1 = flow coefficient
A2 = downstream area of the venturi
Gc = gravitational constant
Rho = density
P1 = inlet pressure
P2 = pressure at venturi minimum area
K2 = K1*A2*SQRT(2*Gc/Rho)

Computing the relationship for volumetric flow rate at the two flow
differentials and equating yields the conversion formula. Note the implicit
assumption that the flow coefficient does not change (it can).

As an example of using the sizing formula, let's say we have a modified 4.1
liter (252 cubic inches) Buick V6 with a VE of 0.9 and we plan to turn no
more than 6400 rpm. Plugging our numbers into the formula yields a
theoretical estimate of:

252 * 6400 * 0.9
CFM = ----------------
3456

= 420 CFM

In practice, Joe Murawski of the Wedge list runs a 4.1L Buick in his Triumph
TR7 and has tried a variety of carbs, in sizes ranging from a Holley 390 to
a 785 CFM Quadrajet, settling on a 500 CFM Edelbrock/Carter AFB as providing
the best power and driveability. His carb choice is somewhat larger than that
predicted. For reasons discussed below, we'll see this is not unusual.

While the formula above may yield useful estimates, it is not necessarily the
ideal it is often portrayed to be. If you have a carb that can flow 500 CFM
in the same application and still properly atomize the fuel, it should make
more power than a 400 CFM carb. From this perspective, larger is better.
Ideally, a carb would present zero restriction to the intake stroke. Such a
carb would have an infinite flow rating. Unfortunately, carbs require a
pressure differential to properly mix fuel with air, which is why carb sizing
is important (and why the above formula is useful). Keep increasing the size
of a carb and, at some point, the booster venturis will not properly atomize
the fuel/air mixture and droplets of liquid fuel will be pulled into the
cylinders. Not only is this bad for performance, it's also hard on the engine.
The liquid fuel tends to wash oil off the cylinder walls, increasing ring
and bore wear. This is a particular problem with engines using large overlap
cams, since they provide lower vacuum levels. When using a larger carb and
cam, proper tuning (carb and ignition) becomes more important.

It's important to understand the basic sizing formula is just a guideline.
It ignores a number of important factors such as manifold design, cam timing,
vehicle weight, gearing, transmission type, intended usage, etc. Furthermore,
it ignores important differences in carb design like venturi efficiency, bore
layout, and secondary style and method of actuation. In practice, I have found
that the above formula applies mainly to square bore carbs with non-air valve
secondaries (Holleys, Autolites), and even then it can be conservative for a
performance application. It typically yields a compromise of fuel efficiency
and power.

Using a dual plane, divided plenum, intake usually allows the use of a carb
with a larger CFM rating than with a single plane, open plenum, intake. This
is because the divider cuts the effective plenum volume in half, increasing
the signal to the boosters. Because of this fact, Edelbrock suggests
multiplying the CFM predicted by the basic sizing formula by 1.1 to 1.3 for
single plane manifolds and by 1.2 to 1.5 for dual planes.

As another example, consider the engine I'm currently running in my Pantera.
It's a 351C Ford with Aussie 2V quench heads, 1 3/4" headers, and a single
plane, open plenum, Weiand Xcelerator intake manifold. Since I retained the
stock cast pistons, I chose a cam with a shift point of 6000 rpm. As a
guess, pick 0.9 for the VE. Since the Pantera is relatively light with short
gearing, pick the high side of the range for K (the intake factor):

K*DISP * RPM * VE 1.3*351*6000*0.9
CFM = ----------------- = ----------------
3456 3456

= 713 CFM

This agrees with real world Pantera club experience with Holleys on street
modified 351C's. 600 CFM carbs provide good throttle response and fuel economy
but give up 20+ peak horsepower to 750 carbs. On the downside, the 750 hurts
fuel economy and has poorer throttle response. The happy medium is probably
somewhere in between. Note we're referring to stock Holley carbs here, not
custom models with milled choke horns, thinned butterflies, and improved
booster designs. Those modified carbs can flow more mixture, while providing
adequate atomization.

I chose a Holley 735 from a 428CJ application which seems to work well. This
carb, while flowing nearly as much as a 750, has a venturi cluster design that
provides a stronger signal. Throttle response and fuel economy are relatively
good (20+ mpg on the highway), without incurring a noticeable power penalty.

Two other important considerations are bore layout and method of secondary
actuation. Carbs with air valve secondaries (Carters and Rochester Quadrajets), especially those with spread bore layouts (Thermo Quads, Quadrajets), can
usually be sized larger than square bore Holleys and Autolites. This is
because the smaller primaries increase the flow speed through the boosters,
providing better atomization, while the air valve secondaries passively
restrict air flow until the engine can handle it. Taking these two factors
into consideration, Vizard suggests the following two rules of thumb for street performance engines where power is more important than fuel economy. For air
valve secondary carbs with an upper rpm limit of 6000 rpm, use:

CFM = 2.3 * DISP

For square bore non-air valve secondary carbs use:

CFM = 2.0 * DISP

For engine speeds above 6000 rpm, multiply by the ratio of maximum rpm to
6000 rpm. Note the second formula yields 702 CFM for my 351C example, which
is close to the basic sizing formula with the intake manifold correction
factor applied.

As an extreme example, I've successfully used a 750 CFM Quadrajet on a
relatively stock 231 cubic inch Buick V6. With the Qjet, it got slightly
better fuel economy than the previous 2 barrel carb (due to the small
primaries) and had noticeably more power (due to the huge secondaries). The
driveability of the carb was fine with no bogs or flat spots. On the V6, I'm
sure it never pulled anywhere near the 750 CFM rating but it did pull what
it required. You could never put a Holley 750 on a little low compression V6
and expect to make it work. The air valve secondaries allow the use of much
larger CFM ratings without incurring driveability problems. There is a price
to be paid however. Even when wide open, air valve secondaries are slightly
more restrictive to airflow than non-air valve secondaries.

While these formulas should help you choose a carb flow rating, nothing beats
trail and error. Also, once you have a carb installed, you can determine how
restrictive it is by using a vacuum gauge to measure the difference between
atmospheric pressure and the pressure under the carb. With the air cleaner
removed, the air above the carb will be essentially atmospheric. If there's
any difference between it and the pressure sensed under the carb, it's due
to the carb. The higher the difference, the greater the restriction.
Measurements should be made at wide open throttle and 0.7 inches of mercury
is considered non-restrictive.

Dan Jones


[This message has been edited by SteveLaRiviere (edited 03-07-2002).]

IP: Logged

Daniel Jones
Gearhead

Posts: 420
From: St. Louis, MO
Registered: Aug 99

posted 03-07-2002 02:01 PM     Click Here to See the Profile for Daniel Jones   Click Here to Email Daniel Jones     Edit/Delete Message   Reply w/Quote
So it looks like you have to do the code function on each section. Oh well. Too much work for me at the moment.

Dan

IP: Logged

SteveLaRiviere
Administrator

Posts: 30097
From: Saco, Maine
Registered: May 99

posted 03-07-2002 02:18 PM     Click Here to See the Profile for SteveLaRiviere   Click Here to Email SteveLaRiviere     Edit/Delete Message   Reply w/Quote
No, you just forgot to open it with the [*code]

I fixed it for you.

------------------

'70 Mustang Mach 1 M code 351C 4V/FMX/3.25 open
'72 Mustang Sprint Coupe 351C 4V/FMX/4.30 Trac Lok
'94 F-150 XL 5.8L/E4OD/3.55 Limited Slip

MCA Member # 47773

An optimist thinks that this is the best possible world. A pessimist fears that this is true.

IP: Logged

Daniel Jones
Gearhead

Posts: 420
From: St. Louis, MO
Registered: Aug 99

posted 03-07-2002 02:51 PM     Click Here to See the Profile for Daniel Jones   Click Here to Email Daniel Jones     Edit/Delete Message   Reply w/Quote
Doh! I hate it when that happens.
Thanks,
Dan Jones

IP: Logged

All times are ET (US)

next newest topic | next oldest topic

Administrative Options: Close Topic | Archive/Move | Delete Topic
Post New Topic  Post A Reply
Hop to:

Contact Us | Mustangsandmore Front Page

Copyright 2003, Steve LaRiviere. All Rights Reserved.


Ultimate Bulletin Board 5.47d

Amazon Honor System Click Here to Pay Learn More

[The M&M Gearhead Gear Store]

[Build a free Mustangsandmore.ws Home Page!]

[About M&M][Acronym Guide][Calendar of Events][Chat Room][Classified Ads] [Links]

[Members' Photos] [M&M Mug Shots] [Technical Articles][Ford Parts Number Deciphering

[ Mustangsandmore.com Bookstore] [Advertise on Mustangsandmore.com] [Mustangsandmore.com T-Shirts]